My theory yesterday was that it was just a typo for 'X'. My theory today is that it was meant to be "DA" for Driving Autopilot/Assist. It is the other thing that has me excited.
Take, for instance, "Efficient Dual Source Battery Pack System for an Electric Vehicle," which is Tesla's patent application # 20120041625. As in "D" Dual source Battery Pack "d" Dual motor drive.